Field of the Invention
The invention relates to a method of determining the relative position of a first imaging or imagesetting device with respect to a second imaging or imagesetting device by setting images on an imaging medium assigned thereto. The invention further relates to a method of correcting the position of a point of projection of a first imaging device with respect to a point of projection of a second imaging device by changing the relative position of the point of projection of the first imaging device with respect to the point of projection of the second imaging device from an actual position to a nominal or reference position, in particular, in a printing form exposer or in a printing unit of a printing press.
In order to set an image on a two-dimensional surface of an imaging medium with the aid of one or more imaging devices, the surface is scanned, in two linearly independent coordinate directions for covering the area thereby, by providing a relative movement between the surface and the imaging device or devices, which is produced by a suitable actuator mechanism. The scanning is typically performed in a so-called fast scanning direction and a so-called slow scanning direction, so that all the points to be imaged on the surface are swept over by the imaging devices, more precisely by a number of imaging beams. In this regard, an imaging beam may be a light beam, in particular a laser light beam, whether in the infrared, visible or ultraviolet spectral ranges, a heat pulse, a gas jet or a droplet of a chemical substance or the like. An imaging device, also referred to as an imaging or imagesetting module, may have one or more imaging beams. In this regard, imaging media include printing forms, printing plates, so-called printing form precursors, films or the like. For setting images on imaging media in the graphics industry, whether at the pre-printing stage in printing form exposers or in the printing stage in printing units (on-press imaging or direct imaging printing units), laser light sources, in particular, are especially widespread in imaging devices. Frequently, laser light sources are diode lasers or solid body lasers, such as lasers with reinforcing media of Ti:sapphire or Nd:YLF, preferably pumped by a diode laser. Several laser light sources may be located on one or more assembled diode laser bars in one imaging device.
An imaging device may include one imaging channel or a group of imaging channels. Several imaging devices may be integrated into a block. For the purpose of imaging or inscribing, the imaging channels are switched on and off (timed releasing). Depending upon the imaging method that is selected, while at least one imaging channel is switched on, a relative movement may or may not take place between the point of projection and the imaging medium. By the use of an imaging medium whereon an image has been set or written, an image can be transferred to a printing material. Typical printing materials are paper, pasteboard or cardboard, organic polymer films or the like, whether they are in sheet or web form.
If a number of imaging devices are used, whether in a printing form exposer or exposing device, or in a printing unit, it is very important that the relative positions of the imaging devices with respect to one another and, assuming that a plurality of imaging beams emanate from one imaging device, that the imaging beams from the imaging device are aligned with one another. If appropriately accurate imaging optics is used on the imaging medium, it is consequently then possible for the relative position of the imaging beams on the imaging medium to be set up or calibrated with great precision relative to one another. As a result, without restricting the general number of imaging beams in an imaging device, merely for simplifying the drawings and description, it is assumed that each imaging device has one imaging beam, respectively. Furthermore, without restricting the general number of the imaging devices, likewise for simplifying the drawings, the description hereinbelow is directed to a first and a second imaging device, with the knowledge that more than two imaging devices may be involved.
The procedure for setting up or calibrating the relative position of a first imaging device with respect to a second imaging device (of a first with respect to a second imaging channel or of a first group of imaging channels with respect to a second group of imaging channels) can be required firstly in the assembly of the imaging devices and secondly in the event of maintenance of the apparatus or the press with which the imaging devices are associated, be it in a workshop or at the customer. According to a widespread procedure, therefore, a considerable amount of effort is associated in particular with the installation of printing presses with on-press imaging printing units. For each printing unit, test imaging of a printing form (imaging medium) is carried out, part of the printing form with an image set on it is cut out and the part is examined by a reader, so that correction values for the relative position can be determined. The correction values are used for making changes in the relative position, for example, this information is made available to the control system, and the imaging devices are adjusted with respect to one another. The aforedescribed method for determining the relative position is iterated until the desired or required precision has been reached. The relative position deviation can be determined both for the fast scanning direction and for the slow scanning direction, and the relative position can consequently be adjusted. However, the high outlay for material and of time is a disadvantage when this method is used.
For example, the published Non-prosecuted German Patent Application DE 44 37 284 A1, corresponding to U.S. Pat. No. 5,832,415, discloses that a calibration of a control system for the deflection of a laser beam can be performed as follows. A light-sensitive medium is irradiated by the laser beam in order to produce a test image and, therefrom, digitized image sections which are recorded by a CNC-controlled camera are then generated. A calculation of correction data for the control system for the diffraction or deflection of the laser beam is performed based upon a comparison between the actual positions of the laser beam, which are measured by recording the image sections, and predefined intended positions. The disadvantage when this method is employed is likewise the outlay for material and is in addition the requirement for using a precise CNC control system for the camera, which is also consequently quite expensive.
German Published, Non-prosecuted Patent Application DE 197 32 668 A1 discloses a calibration device for a beam scanning device, which has a surface with defined markings. With a detector device, the light reflected from the surface or transmitted light from the beam scanning the surface is registered. When the beam strikes a marking, only a low intensity is reflected and transmitted, respectively. From the registered variation in intensity of the radiation in reflection or transmission, the actual position of the laser beam is determined and, in a control system, is compared with the nominal or desired position anticipated for this location. From this comparison, a correction value can be determined and made available to the control system, for example stored as a tabular entry in a memory. A disadvantage of this calibration device is that a surface with precise markings must be made available. However, such a surface is sensitive and furthermore is little suited to be moved to various locations or to be accommodated in different machines, frequently with changes in shape, which may possibly lead to distortions of the coordinate system.
It is accordingly an object of the invention to provide a method for determining the relative position of first and second imaging devices, a method of correcting a position of a point of projection of the devices, a printing form exposer, a printing unit, a printing unit group and a printing press, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and with which it is possible, in a relatively simple manner, to detect a disadjustment of the imaging devices with respect to one another, i.e., a deviation between an actual distance and a nominal or desired distance.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method of determining a relative position of a first imaging device with respect to a second imaging device by setting images on an associated imaging medium. The method comprises, setting an image of a group of mutually different reference patterns and a basic pattern on the imaging medium, through the use of the second imaging device. Each reference pattern from the group thereof is assigned uniquely to one relative position. The first imaging device sets an image of at least one test pattern over the basic pattern, for forming a combination pattern. A reference pattern having an area coverage coinciding with the area coverage of the combination pattern is identified from the group of reference patterns. A relative position associated with the identified reference pattern of the group of reference patterns is identified.
In accordance with another mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes providing that the reference patterns of the group thereof, the basic pattern and the test pattern are two-dimensional and uniform in one of two linearly independent directions.
In accordance with a further mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes providing that the basic pattern is formed of a group of mutually identical control patterns.
In accordance with an added mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes providing that the number of reference patterns in the group thereof and the number of control patterns is the same. A respective control pattern is assigned uniquely to a respective reference pattern in the group of reference patterns.
In accordance with an additional mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes arranging the mutually associated reference patterns in the group thereof and control patterns adjacently on a surface of the imaging medium.
In accordance with yet another mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes providing that the number of reference patterns in the group thereof is uneven. Furthermore, the test pattern covers a first part of the control patterns with a first sub-pattern, and a second part of the control patterns with a second sub-pattern. Both the first sub-pattern and the second sub-pattern cover a control pattern.
In accordance with yet a further mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes providing that the reference patterns from the group thereof, the control patterns and the test pattern are two-dimensional, and uniform in one of two linearly independent directions.
In accordance with yet an added mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes providing that the basic pattern has 50% area coverage.
In accordance with yet an additional mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes providing that the basic pattern has a regular sequence of a plurality of exposed points and a like number of unexposed points in one direction of extent thereof.
In accordance with still another mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes providing that each of the reference patterns in the group of reference patterns has a regular sequence of exposed and unexposed points in one direction of extent thereof. The sequences in one period respectively have a first and a second number of exposed points and a third and a fourth number of unexposed points.
In accordance with still a further mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes providing that the directions of extent of the reference patterns in the group of reference patterns are parallel to one another.
In accordance with still an added mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes providing that the number of reference patterns in the group of mutually different reference patterns is uneven, and assigning a reference pattern to a relative nominal or desired position. This includes assigning a first part of the group of patterns to relative positions which are greater than the relative nominal or desired position, and assigning a second part of the group of patterns to relative positions which are smaller than the relative nominal or desired position.
In accordance with still an additional mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes, for one absolute value of the relative position, providing that the reference pattern from the first part of the group of reference patterns is identical with the reference pattern from the second part of the group of reference patterns.
In accordance with another mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes, for one absolute value of the relative position, providing that the reference pattern from the first part of the group of reference patterns with respect to the reference pattern from the second part of the group is mirror-symmetrical with respect to an axis perpendicular to the extent of one of the reference patterns.
In accordance with a further mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes arranging the group of mutually different reference patterns on the imaging medium for ordering them in accordance with the associated relative nominal or desired position.
In accordance with an added mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device further includes providing for the imaging medium to be accommodated in a printing unit. The imaging medium, whereon an image has been set by the first and the second imaging device, is printed off onto a printing material before identification of the coincident area coverage. An identification is performed on the printing material, and of the relative position associated with the identified reference pattern from the group of reference patterns.
In accordance with an additional mode of the invention, the method of determining the relative position of a first imaging device with respect to a second imaging device includes setting an image on a first imaging medium assigned to the first imaging device and on a second imaging medium assigned to the second imaging device. An image is set from a group of mutually different reference patterns and a basic pattern by the second imaging device on the second imaging medium assigned to the second imaging device. Each reference pattern of the group of reference patterns is assigned uniquely to one relative position. An image of at least one test pattern is set by the first imaging device on the first imaging medium assigned to the first imaging device. The first and the second imaging media, whereon images have been set by the first and the second imaging devices, are printed off onto a printing material in such a manner that a test pattern, of which an image was set by the first imaging device, is printed off over the basic pattern, for producing a combination pattern. A reference pattern is identified from the group of reference patterns, the area coverage of which coincides with the area coverage of the combination pattern. A relative position associated with the identified reference pattern of the group of reference patterns is identified.
With the objects of the invention in view, there is additionally provided a method of correcting a position of a point of projection of a first imaging device with respect to a point of projection of a second imaging device. The method comprises at least one of changing the relative position and changing a timed triggering of the first imaging device with respect to the second imaging device from an actual position to a nominal or desired position after previously determining the actual position. Area coverages are determined by a measuring device.
With the objects of the invention in view, there is furthermore provided a printing form exposer, comprising first and second imaging devices. A controlled actuator mechanism moves the first and the second imaging devices relative to an imaging medium and/or one another. A control unit includes an electronic unit with a memory unit having a computer program stored therein for correcting the position of the first and the second imaging device by the controlled actuator mechanism and/or a provision for changing timed triggering.
With the objects of the invention in view, there is also provided a printing unit having a first and a second imaging device, comprising a controlled actuator mechanism for moving the first and the second imaging device relative to an imaging medium and/or one another. A control unit includes an electronic unit with a memory unit having a computer program stored therein for correcting the position of the first imaging device by the controlled actuator mechanism and/or a provision for changing timed triggering.
With the objects of the invention in view, there is additionally provided a printing unit group, comprising a first printing unit, to which a first imaging device is assigned, and a second printing unit, to which a second imaging device is assigned. A controlled actuator mechanism moves the first and the second imaging devices relative to imaging media and/or one another. A control unit includes an electronic unit with a memory unit having a computer program stored therein for correcting the position of the first imaging device by the controlled actuator mechanism and/or a provision for changing the timed triggering.
With the objects of the invention in view, there is additionally provided a printing press, comprising a printing unit with a first and a second imaging device. A controlled actuator mechanism moves the first and the second imaging device relative to an imaging medium and/or one another. A control unit includes an electronic unit with a memory unit having a computer program stored therein for correcting the position of the first imaging device by the controlled actuator mechanism and/or a provision for changing timed triggering.
With the objects of the invention in view, there is concomitantly provided a printing press, comprising a printing unit group with a first printing unit, to which a first imaging device is assigned, and with a second printing unit, to which a second imaging device is assigned. A controlled actuator mechanism moves the first and the second imaging devices relative to imaging media and/or one another. A control unit includes an electronic unit with a memory unit having a computer program stored therein for correcting the position of the first imaging device by the controlled actuator mechanism and/or a provision for changing the timed triggering.
According to the invention, a determination of the deviation between the position of the first imaging beam, which is associated with the first imaging device, and the position of the second imaging beam, which is associated with the second imaging device, is performed by a tonal value comparison or area coverage comparison between specific patterns, in particular grid or line patterns, directly on the imaging medium whereon an image is set or on a test image printed with the imaging medium. According to the invention, the relative position of the first imaging device (imaging beam) with respect to a second imaging device (imaging beam) which functions as a so-called master, is determined. By a suitable selection of the orientation or direction of the patterns, in particular line grids, registration of the position deviations or beam tolerances in the fast scanning direction and slow scanning direction is possible, the orientation of the patterns, in particular of the direction of the lines, in the case of line grids, preferably being perpendicular to the direction of the dimensional tolerance, of the position deviation. A precondition for the use of the method according to the invention is precise generation of defined, pixel-accurate patterns, in particular line grids, on the imaging medium.
The method according to the invention of determining the relative position of a first imaging device with respect to a second imaging device by setting images on an associated imaging medium includes at least the following steps: an image is set of a group of mutually different reference patterns and a basic pattern by the second imaging device which serves as a master, each reference pattern of the group being assigned uniquely to one relative position. An image of at least one test pattern is set by the first imaging device, the relative position of which is to be determined, over the basic pattern, so that a combination pattern is produced. A reference pattern, from the group, the area coverage of which coincides with the area coverage of the combination pattern is determined. Because the identified pattern from the group is assigned in a unique way to one relative position, the relative position of the first with respect to the second imaging device is identified.
Expressed in other words, the method according to the invention for determining the relative position of two imaging devices (imaging modules or imaging beams) with respect to one another includes setting images over one another or writing patterns over one another, a number of reference patterns being written by the second imaging device, functioning as a master, and at least one test pattern being written by the first imaging device, the relative position deviation of which is to be determined, so that at least one combination pattern is produced for the purpose of comparing the area coverage or the tonal value by an optical route (measurement) either on the imaging medium or on a test image on a printing material printed by using the imaging medium.
The method according to the invention for determining the relative position of a first imaging device with respect to a second imaging device, the first imaging device being associated with a first imaging medium and the second imaging device being associated with a second imaging medium, includes at least the following steps: an image is set of a group of mutually different reference patterns and a basic pattern by the second imaging device on the second imaging medium associated with the second imaging device, each reference pattern from the group being assigned uniquely to one relative position. An image of at least one test pattern is set by the first imaging device on the first imaging medium associated with the first imaging device. The first and second imaging media whereon images are set by the first and the second imaging device are printed off onto a printing material in such a manner that the test pattern, the image of which is set by the first imaging device, is printed off over the basic pattern, so that a combination pattern is produced. A reference pattern, from the group, the area coverage of which coincides with the area coverage of the combination pattern, is identified. Because the identified pattern from the group is assigned a relative position in a unique manner, the relative position of the first with respect to the second imaging device is identified.
It is particularly advantageous if the reference patterns from the group, the basic pattern and the test pattern are two-dimensional, but are uniform or translation-invariant in one of the two linearly independent directions. Expressed in other words, the patterns can have an orientation or direction. The uniformity, in particular for line patterns or line grids, can particularly advantageously extend perpendicularly to the direction of the relative position deviation to be determined, so that precise detection of the relative position in combination patterns is made possible, because even a small deviation in a direction not parallel to the uniformity can lead to a large change in the area coverage of the combination pattern.
The basic pattern can advantageously include a group of mutually identical control patterns. These can be disposed at various locations on the two-dimensional area of the imaging medium. These control patterns can, but do not have to be, overwritten by one and the same test pattern. In the case of a test pattern, the result is redundancy of the information, so that statistical or other error sources can advantageously be avoided. In the case of a plurality of test patterns, different combination patterns are produced, so that additional information can be obtained by optical comparison. It is also advantageous if the number of reference patterns in the group and the number of control patterns are the same and in each case one reference pattern in the group has just one control pattern uniquely assigned thereto. For example, it is additionally possible for the mutually uniquely associated control and reference patterns also to be disposed adjacently on the two-dimensional surface of the imaging medium. By a unique association or an adjacent configuration, an optical comparison can be carried out simply, quickly and conveniently if the control patterns (basic patterns) are covered by test patterns to form combination patterns. In particular, the comparison can be carried out by a single measurement, a first part of the image field being assigned to the combination pattern and a second part of the image field being assigned to the reference pattern.
A further advantageous feature can be formed by the fact that the number of reference patterns in the group and the associated control patterns in the basic pattern can be uneven. It is then possible for the test pattern to cover a first part of the control patterns with a first sub-pattern, to cover a second part of the control patterns with a second sub-pattern and to cover a control pattern both by the first sub-pattern as well as by the second sub-pattern. As a result, at the location of the first sub-pattern, a first combination pattern is then produced, at the location of the second sub-pattern, a second combination pattern is produced and, at the location of the control pattern covered by the first and second sub-pattern, a third combination pattern is produced. The first and the second sub-pattern can be selected so that the first combination pattern is then exactly identical with the associated control pattern covered thereby when the second combination pattern differs from the associated control pattern covered by the latter, and that the second combination pattern is then exactly identical with the associated control pattern covered thereby when the first combination pattern differs from the associated control pattern covered by the latter. The third combination pattern can coincide either with the first or with the second combination pattern. In this way, it is possible to carry out detection of the sign of the relative position between the first and second imaging device. For these additional features, it can also be true that the reference patterns from the group, the control patterns and the test pattern are two-dimensional, but are uniform or translation-invariant in one of the two linearly independent directions.
It is particularly advantageous if the basic pattern exhibits 50% area coverage, so that relative position deviations lead to small area coverage changes in the patterns starting from 50% area coverage. In this range, area coverage changes can be determined easily and well.
In one advantageous embodiment, the basic pattern and/or the control patterns, if the basic pattern includes a group of control patterns, has a regular sequence of a number of exposed and of the same number of unexposed points in one direction of the extent thereof. In other words, a pattern can be a line grid or a strip grid. The lines can be parallel to one another and in particular can run perpendicularly to the direction wherein the relative position is to be determined.
Furthermore, it is also possible for each of the reference patterns in the group of reference patterns to have a regular sequence of exposed and unexposed points in one direction of the extent thereof, the sequences in one period, respectively, having a first and a second number of exposed points and a third and a fourth number of unexposed points. The directions of the extents of the reference patterns in the group of reference patterns can advantageously also be parallel to one another.
It is particularly advantageous if the number of reference patterns in the group of mutually different reference patterns is uneven and if a reference pattern is assigned to the relative nominal or desired position, a first part of the group of reference patterns is assigned to relative positions which are greater than the relative nominal or desired position, and a second part of the group of reference patterns is assigned to relative positions which are smaller than the relative nominal or desired position. In a first embodiment, for one absolute value of the relative position, the reference pattern from the first part of the group can be identical with the reference pattern from the second part of the group. In a second embodiment, for one absolute value of the relative position, the reference pattern from the first part of the group with respect to the reference pattern from the second part of the group may exhibit mirror symmetry with respect to an axis at right angles to the extent of one of the reference patterns.
In a preferred embodiment of the method according to the invention, the group of mutually different reference patterns can be disposed on the imaging medium so as to be ordered in accordance with the associated relative nominal or desired position. Also optional is the imaging or labeling of the reference patterns by a scale. Expressed in other words, the assignment of a reference pattern to a relative position or position deviation is also directly visible to the human eye. An ordered configuration therefore permits quick and simple optical evaluation of the imaging result.
In an advantageous development of the method according to the invention of determining the relative position of a first imaging device with respect to a second imaging device, if the imaging medium is accommodated in a printing unit for the purpose of imaging, provision is made for the imaging medium whereon an image has been set by the first and second imaging device to be printed off onto a printing material before the identification of coincident area coverage, the identification being performed on the printing material, and the identification of the relative position associated with the identified reference pattern from the group are performed. In addition to the particularly convenient and simple possibility of being able to perform measurements of the area of coverage on the printing material and not on the imaging medium accommodated in the printing unit, this development of the method according to the invention also offers the advantage that, if appropriate, influences of the printing unit on the nominal or desired position or relative nominal or desired position of the two imaging beams or imaging devices can be taken into account, because it is ultimately a question of the relative position of a pixel or printed dot placed by the first imaging device in relation to the position of a pixel or printed dot placed by the second imaging device in the image on the printing material.
The method according to the invention, the advantageous embodiments thereof or the advantageous developments thereof avoid great expenditure of labor and time, because processing of the imaging medium, printing form material, outside the printing form exposer or the printing unit, and examination under a reader or aids, such as magnifying glasses, imaging optics or the like and renewed loading of an imaging medium are no longer necessary, in contrast with the prior state of the art.
The method according to the invention of determining the relative position or the relative position deviation of two imaging devices can be developed further to a method of correcting the position of a first imaging device with respect to a second imaging device, and more accurately, to correcting the position of the points of projection of a first imaging device with respect to points of projection of a second imaging device, by changing the relative position of the first imaging device with respect to the second imaging device from an actual position to a nominal or desired position and/or by changing the timed triggering (of the on and off switching). In this method according to the invention for correcting the position of the points of projection, the area coverages are determined by a measuring device, for example a photodiode or a CCD array, by the intensity of the light reflected from the combination patterns and the reference patterns being detected and the measured values being compared in a suitable processing unit. As a result, the identification of combination patterns and reference patterns is made possible in electronic form, so that a signal for changing the relative position of the first imaging device with respect to the second imaging device and, consequently, also for changing the relative position of the associated points of projection can be generated. On the one hand, this signal can be in the form of a display for a human being, on the other hand, provision can also be made for the relative position to be changed from an actual position to a nominal or desired position by an actuator mechanism. As an alternative thereto, the timed triggering of the first imaging device relative to the second imaging device can take place later or earlier than in the uncorrected state.
The method according to the invention for correcting the position of the points of projection can be implemented in the following manner in a device: a printing form exposer having at least a first and a second imaging device, which can be moved relative to the imaging medium and/or to one another by a controlled actuator mechanism, has a control unit. The printing form exposer is distinguished by the fact that the control unit includes an electronic unit with a memory unit, wherein a computer program for correcting the position of the first imaging device by the controlled actuator mechanism and/or by changed timed triggering is stored, the computer program having at least one functional section wherein steps from the method according to the invention for correcting the position by the actuator mechanism and/or the timed triggering are performed.
As an alternative thereto, an implementation can also be performed for a printing unit or a printing unit group: a printing unit having a first and a second imaging device or a printing unit group having a first printing unit, to which a first imaging device is assigned, and having a second printing unit, to which a second imaging device is assigned, the imaging devices, which are movable relative to the imaging medium and to the imaging media and to one another, respectively, by a controlled actuator mechanism, having a control unit. The printing unit and the printing unit group, respectively, are distinguished by the fact that the control unit includes an electronic unit with a memory unit, wherein a computer program for correcting the position of the first imaging device by the controlled actuator mechanism and/or changing the timed triggering is stored, the computer program having at least one functional section wherein steps from the method according to the invention for correcting the position by the actuator mechanism and/or changing the timed triggering are performed. A printing press according to the invention, whether it is a web-fed or a sheet-fed printing press, in particular a planographic printing press, an offset printing press or the like, has at least one printing unit according to the invention and/or at least one printing unit group according to the invention.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for determining the relative position of first and second imaging devices, a method of correcting a position of a point of projection of the devices, a printing form exposer, a printing unit, a printing unit group and a printing press, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.